A lambda regulation in connection with a catalyzer is nowadays the most efficient exhaust gas purification system for the Otto engine. Only together with nowadays available ignition- and injection systems very low exhaust gas values can be achieved.
Particularly efficient is the use of a three-way or selective-catalyzer. That catalyzer has the feature to reduce hydrocarbons, carbon monoxide and nitrous gases up to more than 98%, if the engine is operated in a range of approximately 1% around the stoichiometric air fuel relation with λ=1. The lambda value provides thereby how far the actually present air fuel mixture deviates from the value λ=1, which is equivalent to a mass relation of 14.7 kg air to 1 kg benzene that is theoretically necessary for a complete combustion, which means that the lambda value is the quotient of the supplied air mass and theoretical air demand. At an air surplus λ is >1 (lean mixture). At a fuel surplus λ is <1 (rich mixture).
Each exhaust gas is basically measured at a lambda regulation and the supplied fuel amount is instantly corrected according to the measuring result for example with the aid of an injection system.
As a sensor element lambda probes are used, which can be construed on the hand as so-called two-point lambda probe or snap probe and on the other hand as constant lambda probe or wideband lambda probe. The effect of these lambda probes is based on the familiar principle of a galvanic oxygen concentration cell with a solid electrolyte. The characteristic line of a two-point lambda probe provides a jerky drop o the probe voltage at λ−1. Therefore a two-point lambda probe, which is usually directly attached behind the exhaust gas manifold, enables basically only the distinction between rich and lean exhaust gas. Both lambda probe types consist of a ceramic sensor element, a protection pipe and as cables, a plug and the connections between those elements. The protection pipe consists of one or several metal cylinders with openings. Through these openings exhaust gas enters by diffusion or convection and gets to the sensor element. The sensor elements of the two lambda probe types are therefore construed differently.
Significant for the low-emission operation of the combustion engine is a quick regulation of the exhaust gas composition on to the preset lambda value. This applies in particular also to combustion engines with a single cylinder regulation, at which the air-fuel mixture is adjusted individually for each single cylinder of the combustion engine on the basis of the signal of the common lambda probe. Therefore the lambda measurement has to take place with a high temporal resolution in order to determine the consecutive exhaust gas volumes of the different cylinders in its composition that get to the lambda probe and to be able to assign them to a corresponding cylinder.
Besides the selected regulation parameters of the lambda control loops and the distance parameters the dynamic of the lambda probe determines the speed of the control loop. The dynamic of the lambda probes in mint condition is thereby also sufficient for a single cylinder regulation with one lambda probe in a common exhaust gas duct for all cylinders. Due to ageing effects the dynamic characteristics of the lambda probes can be changed in such a way that the temporal resolution of the determination of the exhaust gas composition is not sufficient anymore, which causes an increased oxygen emission. If that is located outside the statutory provisions the lacking dynamic of the lambda probe has to be determined in the range of the on-board diagnosis of the combustion engine and a corresponding error message has to be provided. In several countries the statutory provisions for motor vehicles require that such a diagnosis has to be implemented in engine control units, which turns on an error light when the lambda probe slows down, which causes an exceeding of a determined pollution threshold value. In the USA the dynamic parameter that has to be monitored is précised as the so-called response-time, which means the time between a change of the oxygen or rich gas concentration in the exhaust gas at the probe and the corresponding change of the probe signal.
The state of the art knows a number of diagnosis procedures, for example the comparison of a measured and an expected lambda signal at a familiar stimulus.
The application DE 102 60 721 A1 describes for example a procedure for diagnosing the dynamic features of a lambda probe, which is used at least temporally for a cylinder individual lambda regulation, as well as an associated diagnosis device. It is thereby provided that at least one correcting variable of the lambda regulation is detected and compared to a default maximum threshold and in the case of an exceeding of the maximum threshold the dynamic behavior of the lambda probe is evaluated as not sufficient with regard to the operational capability for the cylinder individual lambda regulation. The dynamic features of the lambda probe can be detected from the single cylinder regulation itself because the cylinder individual regulators do not diverge at an insufficient dynamic of the lambda probe. Furthermore the test function can provided with an aimed disorder or alienation of the actual lambda value. The procedure is therefore suitable for combustion engines with a single cylinder lambda regulation or it requires an aimed influencing of the lambda value.
But several procedures have the disadvantage that only a change of the temporal constant of the lambda probe but nit a pure dead time in the probe signal can be detected. It is for example not possible to detect a dead time at a comparison between measured and expected lambda signal at a periodical stimulus because there is no possibility to decide whether an observed reaction in the measured lambda signal is based on the stimulus of the directly preliminary period or an earlier period.
It is therefore the task of the invention to provide a procedure, which enables a reliable and improved on-board diagnosis of the dynamic of an exhaust gas sensor. It is furthermore the task of the invention to provide a corresponding device.